Extruding vinylidene chloride copolymer flexible packaging film

ABSTRACT

This invention relates to extruding films made from vinylidene chloride copolymers. Specifically, this invention relates to a method for lowering the oxygen permeability and enhancing the thermal stability and enhancing the melt shear stability of packaging films having a layer of vinylidene chloride copolymer film by adding a processing aid such as an epoxy resin to vinylidene chloride copolymer of low molecular weight. The conventional stabilizer/plasticizer combination of epoxy resin and 2-ethyl-hexyl diphenyl phosphate may be employed with the vinylidene chloride copolymer. The layer of vinylidene chloride copolymer can be as thin as 0.18 mil.

This is a Divisional of Application Ser. No. 364,664, filed on Jun. 9,1989, now U.S. Pat. No. 5,030,511, issued Jul. 9, 1991.

BACKGROUND OF THE INVENTION

This invention relates generally to vinylidene chloride copolymerscontaining a processing aid and flexible film produced therefrom. Moreparticularly, vinylidene chloride copolymers of low molecular weight,with a processing aid preferably also of low molecular weight, have beensurprisingly found to be of enhanced thermal stability. Thus, they canbe extruded into film at a higher temperature, or at conventionaltemperatures but with less processing aid. The vinylidene chloridecopolymer layer in the extruded film can be very thin, as thin as 0.18mils, yet the film will still possess excellent oxygen barriercharacteristics exhibiting an oxygen permeability under about 8cc/mil/m² /day/atmosphere. This is equivalent to an oxygen transmissionrate under about 44.44 cc/sq m/day atmosphere for this film having a0.18 mil gauge vinylidene chloride copolymer layer.

Flexible thermoplastic films made of vinylidene chloride copolymer,hereinafter referred to generally as PVDC (such materials are alsocommonly known as "saran", which, in the United States has becomegeneric and is not a registered trademark) have been used for a longtime to package food products which include cheese, fresh and processedmeats, and a wide variety of other food and non-food items. PVDC is abarrier to oxygen and thus such films protect food from spoilage.

Processing aids are standard practice for PVDC to be successfullyextruded at commercial rates, i.e. the vinylidene chloride copolymersmust be stabilized and plasticized. A successful plasticizer-stabilizercombination is the liquid stabilizer, epichlorohydrin/bisphenol A, anepoxy resin sold as EPON resin 828 by the Shell Chemical Company, andthe plasticizer, 2-ethyl hexyl diphenyl phosphate sold as Santicizer-141by Monsanto Chemical Co. Other known liquid stabilizers include epoxycompounds such as epoxidized linseed oil and epoxidized soybean oil andcitrates. A quite successful and satisfactory plasticizer/stabilizerpackage is made using approximately 4% of Epon 828 and approximately 2%of Santicizer-141 in PVDC. For clarity, it is noted that sometimes theart refers to the epoxy compounds as plasticizers instead of asstabilizers.

A method of producing a multilayer film having a PVDC layer is disclosedin U.S. Pat. No. 4,112,181, issued on Sep. 5, 1978 to Baird, Jr. et al.This patent describes a method of coextruding a tubular film wherein thewalls of the tube have at least three layers, a center layer being aPVDC layer. The tubular film is subsequently biaxially oriented by thetrapped bubble technique. The 3-layer film may be cross-linked byelectron beam irradiation.

Another satisfactory method of producing a multilayer saran film isdisclosed in U.S. Pat. No. 3,741,253, issued on Jun. 26, 1973 to Brax etal, which discloses a multilayer, biaxially oriented film having a PVDCbarrier layer. This film is made by an extrusion coating process inwhich a substrate layer or layers of a polymer such as polyethylene orethylene vinyl acetate copolymer is extruded in the form of a tube,cross-linked by irradiation, and inflated. A layer of PVDC is extrusioncoated onto the inflated tubing, and another layer or layers of polymeris simultaneously or sequentially extrusion coated onto the PVDC. Aftercooling, this multilayer tubular structure is flattened and rolled up.Then, the tube is inflated, and heated to its orientation temperature,thereby biaxially orienting the film. The bubble is rapidly cooled toset the orientation. This process produces a heat shrinkable barrierfilm with low oxygen permeability. Also, the advantages of across-linked film are provided without subjecting the PVDC layer toirradiation which tends to degrade saran. The barrier layer in theexamples of the patent to Brax et al is a plasticized copolymer ofvinylidene chloride and vinyl chloride.

In Canadian Patent No. 968,689, issued on Jun. 5, 1975 to Gillio-tos etal, the effect of plasticizers such as dibutyl sebacate on the barrierproperties of a PVDC barrier layer in a multilayer thermoplasticpackaging film is described. First, the Gillio-tos et al patentdiscloses that homopolymers of vinylidene chloride cannot be convertedinto film by conventional extrusion techniques because they decomposevery rapidly at the temperature of extrusion. Second, by copolymerizingvinylidene chloride with minor amounts of one or more other monomerssuch as vinyl chloride, methyl acrylate, methyl methacrylate,acryonitile, etc. it is possible to produce copolymers which, when mixedwith suitable plasticizers, can be extruded into films which can beoriented by stretching to give heat shrinkable film. The oriented, heatshrinkable, PVDC films are widely used for packaging purposes,particularly for packaging food. As stated in Gillio-tos et al,vinylidene chloride copolymers need to be plasticized so that they canbe extruded and stretched into oriented films at commercial rates. Thegreater the proportion of plasticizer, the lower the viscosity and theeasier the polymer is to extrude and orient and the better the abuseresistance of the final product. On the other hand, the oxygentransmission rate of the final product also increases with increasingplasticizer content and for many purposes, especially packaging food, itis vital that the oxygen transmission rate is low. In recent years, thepackaging industry has become increasingly demanding and for currentcommercial practices an oxygen transmission rate below 100 cc/24hours/m² /atmosphere at room temperature is expected and below 50 ishighly desirable. The test for oxygen transmission is conducted as perASTM D3985.

Also, as the thickness of the vinylidene chloride copolymer layerdecreases, the oxygen transmission rate will increase. Thus, for thinfilms, the oxygen transmission rate still should meet the standard ofless than 100, preferably less than 50 cc/24 hrs/sq m/atm. As mentionedabove, the thin films (0.18 mil vinylidene chloride copolymer layer) ofthe invention do.

Of interest is U.S. Pat. No. 4,714,638 (Dec. 22, 1987) to Lustig et al,assignors to Viskase. This patent discloses heat-shrinkable, biaxiallystretched multi-layer film having a barrier layer of PVDC wherein thecomonomer of the PVDC is methyl acrylate in an amount of 5 to 15% byweight. This patent discusses using conventional plasticizers for thePVDC-MA, such as dibutyl sebacate or epoxidized soybean oil. Similar toU.S. Pat. No. 4,714,638 is U.S. Pat. No. 4,798,751 (Jan. 17, 1989),Schuetz, also assigned to Viskase, wherein the PVDC layer of thebiaxially stretched, heat-shrinkable multi-layer film is a blend of2.9-13.5% PVDC-MA and 2.9-11.6% PVDC-VCl. This latter patent alsodiscusses dibutyl sebacate and epoxidized soybean oil.

Also of interest is U.S. Pat. No. 4,320,175 issued Mar. 16, 1982 toHisazumi et al assignors to Kureha, which shows a PVDC layer composed ofPVDC of 0.030 to 0.050 reduced viscosity heat-pressure laminated to aPVDC layer composed of PVDC of 0.050 to 0.075 reduced viscosity. Epoxycompounds are used as stabilizers for the PVDC.

Also of interest are the following patents which show various additivesfor PVDC. U.S. Pat. No. 4,401,788, issued Aug. 30, 1983 to Hiyoshi etal, assignors to Asahi Dow, shows a PVDC latex with an anionicsurfactant, a nonionic surfactant, and a catonic surfactant. U.S. Pat.No. 4,360,612, issued Nov. 23, 1982 to Trumbull et al, assignors to Dow,shows extruded PVDC film containing an alkali metal salt of an alkylarylsulfonic acid. U.S. Pat. No. 4,418,168, issued Nov. 29, 1983 to Johnson,assignor to Dow, shows stabilizing particulate PVDC by coating thereon adispersion of tetrasodium pyrophosphate (TSPP) in liquid plasticizersuch as epoxidized soybean oil and epoxidized linseed oil. U.S. Pat.Nos. 2,205,449, issued Jun. 25, 1940 and 2,320,112, issued May 25, 1943,both to Wiley, assignor to Dow, show PVDC plasticized withphenoxypropylene oxide (lines 48-49, column 1 of '449) and PVDCplasticized with di-(alpha-phenyl-ethyl) ether (line 16, column 2 of'112). German Patent 3,605,405, priority Feb. 20, 1986, published Dec.12, 1987, shows 5-layer films with a PVDC core layer, wherein the EVAadhesion layers on each side of the PVDC core layer contain TSPP to heatstabilize the PVDC core layer. U.S. Pat. No. 3,524,795, issued Aug. 18,1970 to Peterson, assignor to Dow, shows multiple layer PVDC films andmentions at lines 55-59 of column 4 typical plasticizers for PVDC beingacetal tributyl citrate, epoxidized soybean oil, and dibutyl sebacate.

Among the plasticizers for thermoplastics which are listed in generalarticles and in literature references is glycerol or glycerin. Glycerintogether with the above-mentioned epoxy resins as a plasticizercombination for PVDC is disclosed in U.S. Pat. No. 4,686,148 to Havens.

An object of the present invention is to improve the thermal stabilityof vinylidene chloride copolymers and to lessen their tendency todegrade while being extruded, and hence provide improved melt shearstability. An advantage is that PVDC can be extruded at highertemperatures. Alternatively, the PVDC can be extruded at the sameconventional temperature, but with less additive to achieve thermalstability. The less the additive is, the lower the oxygen permeabilityis. Thus another advantage is that films with a thinner PVDC layer canbe used and they will have a low oxygen permeability comparable to filmswith a thicker PVDC layer (and more additive). The after orientationthickness (after biaxial stretching) can be as low as less than about0.29 mils, preferably less than about 0.26 mils, more preferably lessthan about 0.23 mils, most preferably less than about 0.20 mils. Thelimit thickness is about 0.08 mils.

SUMMARY OF INVENTION

The present invention provides for an extrudable vinylidene chloridepolymeric composition comprising:

(a) about 0.01% to 6% by weight of processing aid, and

(b) low weight-average molecular weight vinylidene chloride copolymerwherein the major portion of the copolymer is vinylidene chloride.

The present invention also provides for a multilayer polymeric filmcomprising:

(a) first and second layers; and

(b) a barrier layer disposed between said first and second layers, saidbarrier layer comprising

(1) about 0.01% to 6% processing aid by weight, and

(2) low weight-average molecular weight vinylidene chloride copolymerwherein the major portion of the copolymer is vinylidene chloride.

Optionally, this multilayer film may be irradiated by electron beam at adosage of about 0.5 to about 6.0 MR with minimal or no discoloration tothe vinylidene chloride copolymer layer.

The invention also provides that in a flexible polymeric film ofvinylidene chloride copolymer wherein the major portion of the copolymeris vinylidene chloride, said vinylidene chloride copolymer containing aprocessing aid, the improvement comprising the vinylidene chloridecopolymer is of a weight-average molecular weight of about 100,000 orless. Preferably, when this film is stretch oriented, the vinylidenechloride copolymer layer has a thickness less than about 0.29 mil, andcan be as low as about 0.08 mil. A very desirable thickness is 0.18 mil,and at this thickness the film will exhibit an excellent oxygentransmission rate less than about 45 cc/sq m/24 hrs/atmosphere.

The invention also provides a multi-layer biaxially stretchedheat-shrinkable film comprising a first layer of ethylene vinyl acetatecopolymer having a melt index greater than about 1.2, a second layer ofethylene vinyl acetate copolymer having a melt index greater than about1.2, and disposed therebetween a core layer of vinylidene chloridecopolymer, said vinylidene chloride copolymer layer being on eachsurface in direct surface-to-surface contact with each of said first andsecond ethylene vinyl acetate copolymer layers, wherein the majorportion of the vinylidene chloride copolymer is vinyl chloride, and saidvinylidene copolymer layer contains a processing aid of molecular weightless than about 700, and said vinylidene chloride copolymer is ofweight-average molecular weight less than about 100,000, and saidvinylidene chloride copolymer layer has a thickness less than about 0.29mils. Most preferably, the thickness is not greater than about 0.20mils.

The invention also provides that in a method of improving the heatstability of vinylidene chloride copolymer during tubular extrusion witha hot blown bubble followed by biaxial stretch orientation, theimprovement comprising extruding a film with a layer of vinylidenechloride copolymer blend comprising: (a) about 0.01% to 6% by weight ofprocessing aid, (b) low weight-average molecular weight of about 100,000or less vinylidene chloride copolymer wherein the major portion of thecopolymer is vinylidene chloride, and (c) the vinylidene chloridecopolymer blend layer has a thickness after orientation less than about0.29 mils.

DETAILED DESCRIPTION

It has been surprisingly discovered that the addition of processing aidsto low weight-average molecular weight vinylidene chloride copolymersprovides several improvements to extrusion of vinylidene chloridecopolymers and films produced thereby. Preferably, the processing aid isalso of low molecular weight. The enhanced thermal stability allowsreduction of conventional additives, i.e. plasticizers/stabilizers, thereduction of which improves oxygen barrier properties. Furthermore, theimproved thermal stability results in generation of less shear heatduring processing which leads to less vinylidene chloride copolymerdegradation. This allows reduction or modification of the additivesnormally required to prevent heat-induced degradation which, in turn,can lead to further oxygen barrier improvements. Also, an increase inthermal stability is directly related to an increase in melt shearstability. Therefore, the sum of these effects permits extrusion speedsand orientation rates to be maintained with improvements in oxygenbarrier properties.

Suitable polyvinylidene chloride for use in the present invention iscommercially available. For instance, Dow Chemical sells PVDC-MA, andSolvay sells PVDC-VCl and PVDC-MA.

It is known that if a multilayer film containing a PVDC layer iselectron beam irradiated, the PVDC tends to darken and degrade. Anotherbenefit of the improved barrier characteristics resulting from thepresent invention is that a thinner PVDC layer can be used. When athinner PVDC barrier layer is used, there is less PVDC to expose toradiation if an irradiation cross-linked multilayer film is desired.Since with the present invention the mass of PVDC exposed to irradiationis reduced, numerous cross-linked film combinations are available whichwere not heretofore feasible. In other words, a coextruded film with athin PVDC layer can be post irradiated with no or minimal discolorationto the PVDC layer.

Common methods for determining overall thermal stability for extrusionof vinylidene chloride copolymer blends with additives such asplasticizers and stabilizers involve working the blend between a pair ofheated rollers or inside a heated mixing chamber. The time required forshear and temperature-induced degradation to produce a noticeablyblackened polymer is a measure of effectiveness of additives such as aplasticizer/stabilizer combination in promoting heat stability.Commercially acceptable vinylidene chloride copolymer additivecombinations should show thermal stability times of 10-15 minutes orbetter in a mixing chamber such as a Brabender plasticorder at 300° F.(149° C.) to 330° F. (165° C.).

It has been unexpectedly found that when low weight-average molecularweight PVDC is employed, the addition of a processing aid increasesthermal stability from around 12 minutes for conventional high molecularweight PVDC to around 20 minutes for low molecular weight PVDC. Theweight average molecular weight of the PVDC should be not greater thanabout 100,000, more preferably not greater than about 95,000, even morepreferably not greater than about 85,000. Furthermore, additionalimprovement of 26+ minute thermal stability can be achieved when lowweight-average molecular weight PVDC has added thereto a processing aidwhich is also of low molecular weight not greater than about 700, morepreferably not greater than about 500 molecular weight.

Thus, in one aspect, the present invention is an extrudable vinylidenechloride polymeric composition comprising by weight about 0.01% to 6.0%,preferably about 0.1% to 5.0%, more preferably about 0.2% to 4.0%,processing aid with the balance being vinylidene chloride copolymer ofabout 100,000 weight-average molecular weight or less. Preferredprocessing aids include, but are not limited to, epoxidized compounds,such as epoxidized linseed oil, expoxidized soybean oil,epichlorohydrin/bisphenol A, epoxidized octyl tallate, epoxidized glycoldioleate, butyl ester of epoxidized linseed oil fatty acid, and thelike, which may be included in quantities up to about 6% by weight.Other suitable processing aids may include an additive such as 2-ethylhexyl diphenyl phosphate, tetrasodium pyrophosphate, oxidizedpolyethylene, antioxidant, magnesium oxide, or chlorinated polyethylene.

In another aspect, the present invention is a multilayer polymeric filmcomprising first and second polymeric layers with a vinylidene chloridecopolymer layer, containing up to about 6% by weight processing aid,disposed between said polymeric layers, in which the vinylidene chloridecopolymer is of weight-average molecular weight of about 100000 or less.Also, (a) a film layer may be irradiated followed by extrusion coatingthe PVDC and another film layer followed by electron beam irradiation ofthe entire multilayer film, or (b) the multilayer film may be coextrudedfollowed by electron beam irradiation of the entire multilayer film.Optionally, the multilayer film may be stretch oriented to make it heatshrinkable either before or after irradiation.

Irradiation of the entire multilayer film or a layer thereof may beaccomplished by the use of high energy electrons. Preferably, electronsare employed up to about 6 megarads (MR) dosage level. The irradiationsource can be any electron beam generator operating in a range of about150 kilovolts to about 6 megavolts with a power output capable ofsupplying the desired dosage. The voltage can be adjusted to appropriatelevels which may be for example 1,000,000 or 2,000,000 or 3,000,000 or6,000,000 or higher or lower. Many apparatus for irradiating films areknown to those of skill in the art. The irradiation is usually carriedout at a dosage up to about 6 MR, typically between about 0.5 MR andabout 6.0 MR, with a preferred dosage range of about 1 MR to about 4 MR.Irradiation can be carried out conveniently at room temperature,although higher and lower temperatures, for example, 0° C. to 60° C. maybe employed.

In still another aspect, the present invention is a method of loweringthe oxygen permeability and improving the thermal stability of avinylidene chloride copolymer film comprising the steps of preparing amixture comprising a vinylidene chloride copolymer of about 100,000 orless weight-average molecular weight and about 0.01% to 6%, preferablyabout 0.1% to 5.0%, by weight processing aid, blending said mixture; andthereafter extruding a film from the mixture. More preferably, theprocessing aid is present as about 0.2% to 4.0% by weight, and theweight-average molecular weight of the vinylidene chloride is about95,000 or less.

Still other aspects of the present invention include irradiation of amultilayer film, which has a processing aid in the saran layer asspecified above, to cross-link the cross-linkable layers. Suchcross-linkable layers may be surface layers or internal layers inaddition to the saran layer and are preferably polyolefins selected fromthe group consisting of ethylene-vinyl acetate copolymer (EVA), branchedpolyethylene (PE), linear low density and very low density polyethylene(LLDPE and VLDPE), low density polyethylene (LDPE), ethylene-butylacrylate copolymer (EBA), ethylene-propylene copolymer (EPC), highdensity polyethylene (HDPE) and blends thereof. Suitable EVAs have amelt index (MI) above about 1.0, more preferably above about 1.2, evenmore preferably above about 1.5 decigram/minute. These include, but arenot limited to LD722.62 supplied by Exxon, which is EVA (20% VA) withMI=3.0; LD318.92 supplied by Exxon, which is EVA (9% VA) with MI=2.0;Elvax 3128 supplied by duPont, which is EVA (9% VA) with MI=2.0; andNA-295-00 supplied by USI, which is EVA (6% VA) with MI=2.6. SuitableLLDPEs may be purchased from Dow Chemical, which include but are notlimited to, Dowlex 2045.04, which has density=0.918 g/cc and MI=1.1 andcomonomer=octene and Dowlex 2045.03, which has density=0.920 g/cc andMI=1.1 and comonomer=octene.

Any saran processing aid or mixtures thereof may be employed in thepresent invention, although processing aids which possess a thermallystabilizing influence defined by the five characteristics outlined inthe Encyclopedia of Polymer Science and Technology, Volume 14 (1971),pages 174-175, are preferred. These characteristics are:

1. adsorb, or combine, with hydrochloric acid gas in an irreversiblemanner under the condition of use, but not have such strong affinity asto strip HCl from the polymer chain,

2. act as a selective ultraviolet light absorber to reduce the totalultraviolet energy absorbed in the polymer,

3. contain a reactive dienophilic molecule capable of destroying thediscoloration by reacting with, and breaking up, the color-producing,conjugated polyene sequences,

4. possess anti-oxidant activity in order to prolong the inductionperiod of the oxidation process and prevent the formation of carbonylgroups and other chlorine labilizing structures resulting from oxidationof polymer molecules,

5. have the ability to chelate metals, such as iron, and prevent theformation of metallic chloride which acts as a catalyst for polymerdegradation.

Preferred ones include, but are not limited to those processing aidsthat have an oxirane (epoxy) ##STR1## functionality. While it is notintended to be bound to any theory it is believed that thisfunctionality reacts with the HCl liberated during dehydrochlorinationof the PVDC molecular chain, thus binding both the hydrogen and chlorideions so as to slow the rate of saran degradation during saran meltingand extrusion. Following reaction with HCl, the epoxy functionally isexpected to form

    HO--C--C--Cl

thus tying up the Cl which then is not available to degrade the saranduring heating in the extruder. However, lower molecular weightprocessing aids work even better. While it is not intended to be boundto any theory, it is believed that the lower molecular weight processingaids work even better since mobility within the polymer is enhanced,since molecular weight is an indicator of molecular size. Also as theweight-average molecular weight of the PVDC is decreased, free volumewithin the polymer matrix is increased, an effect which it is believedalso facilitates processing aid mobility. Epoxidized compound haveoxirane functionalities and thus preferred processing aids areepoxidized compounds such as epoxidized linseed oil, epoxidized soybeanoil, epichlorohydrin/bisphenol A, epoxidized tallate, epoxidized glycoldioleate, butyl ester of epoxidized linseed oil fatty acid and the like.Other processing aids may include an additive such as 2-ethyl hexyldiphenyl phosphate, tetrasodium pyrophosphate, oxidized polyethylene,antioxidant, magnesium oxide, or chlorinated polyethylene. Suitableepoxy compounds for use in the present invention may be purchased fromC. P. Hall Company or Viking Chemical Company. Brochures entitled"Technical Data" from C. P. Hall Company describe their registeredtrademark Monoplex S-73 and Monoplex S-75, which are epoxy plasticizersfor polyvinyl chloride (PVC). A brochure entitled "Vikoflex" from VikingChemical Company describes their Vikoflex epoxy plasticizers and estersfor PVC.

DEFINITIONS

As used herein, the following terms are understood to have the meaningset forth below:

"Polymer" means the product of polymerization and includes but is notlimited to homopolymers, monopolymers, copolymers, interpolymers,terpolymers, block copolymers, graft copolymers, and additioncopolymers.

"Processing aid" means a substance or material incorporated in a film orfilm layer to increase the flexibility, workability, or extrudability ofthe film. These substances include both monomeric plasticizers andpolymeric plasticizers and are generally those materials which functionby reducing the normal intermolecular forces in a resin thus permittingthe macromolecules to slide over one another more freely. The art refersto many plasticizers as stabilizers. Thus, the terms "plasticizer" and"stabilizer" are intended to be used interchangeably herein.

"Oriented" or "Orientation" refer to the process of stretching a hotplastic article followed by rapidly cooling while in the stretchedcondition to realign a molecular configuration thus improving mechanicalproperties. Stretching in one direction is called uniaxial orientationand in two directions is called biaxial orientation. In thermoplasticflexible films which have been oriented there is an internal stressremaining in the plastic sheet which can be relieved by reheating thesheet to a temperature above that at which it was oriented. The materialwill then tend to shrink back to the original dimensions it had beforeit was stretch oriented. Thus "oriented" flexible films are"heat-shrinkable" flexible films, and the terms "oriented" and"heat-shrinkable" are used interchangeably herein. For clarity, it isnoted that films made by a tubular process are referred to as having anorientation along the length of the tube, called the longitudinaldirection (abbreviated herein as L) and/or across the width of the tube,called the transverse direction (abbreviated herein as T).

An "oriented" or "heat shrinkable" material is defined herein as amaterial which, when heated to an appropriate temperature above roomtemperature (for example 96° C.), will have a free shrink of about 5% orgreater in at least one linear direction, as per ASTM D 2732.

"Melt index", abbreviated herein as MI, means melt flow measured at 190°C., 2.16 kilogram loading, as per ASTM D 1238, condition E.

"Vinylidene chloride polymer" or "vinylidene chloride copolymer" or"saran" or "PVDC" means vinylidene chloride copolymerized with at leastone other monomer which includes, but is not limited to, vinyl chloride,C₁ to C₈ alkyl acrylates (such as methyl acrylate), C₁ to C₈ alkylmethacrylates, and acrylonitrile. As abbreviations employed here, PVDCis used to designate copolymers of vinylidene chloride, PVDC-MAdesignates vinylidene chloride/methyl acrylate copolymer and PVDC-VCldesignates vinylidene chloride/vinyl chloride copolymer.

As used herein the term "extrusion" or the term "extruding" is intendedto include coextrusion, extrusion coating, or combinations thereof,whether by tubular methods, planar methods, or combinations thereof.

"Barrier" refers to a property in thermoplastic materials whichindicates that the particular material has a very low permeability togases, such as oxygen. The principal barrier materials referred toherein are the vinylidene chloride copolymers designated as "PVDC".Other known barrier materials are hydrolyzed ethylene-vinyl acetatecopolymers designated by the abbreviations: "EVAL" or "EVOH" or "HEVA",and polyamides, also known as nylons. The inventive film may optionallyhave one or more layers comprising EVOH or polyamide.

    ______________________________________                                        ABBREVIATIONS AND MATERIALS EMPLOYED IN                                       EXAMPLES                                                                      Designation                                                                              Description                                                        ______________________________________                                        RH         Relative humidity                                                  psi        Pounds per square inch                                             ft-lbf     Foot-pounds force                                                  Sq in      Square inch                                                        Sq m       Square meter                                                       rpm        Rotations per minute                                               OTR        Oxygen transmission rate                                           L          Longitudinal direction of tubular film                             T          Transverse direction of tubular film                               (WA) MW    (Weight-Average) Molecular weight                                  PVDC       Vinylidene chloride copolymer                                      PVDC-MA    A copolymer of vinylidene chloride                                            with methyl acrylate sold by Dow                                              Chemical Company. It is about 91.5%                                           VDC and about 8.5% MA by weight.                                   PVDC-MA(1) Dow XU32034.00. (WA) MW = 85,000.                                  PVDC-MA(2) Dow XU32036.00. (WA) MW = 95,000.                                  PVDC-MA(3) Dow XU32027.01. (WA) MW = 105,000.                                 PVDC-VCl   A copolymer of vinylidene chloride                                            with vinyl chloride wherein (WA)                                              MW = 85,000. It is about 91.5% VDC and                                        8.5% VCl by weight.                                                EPOXY(1)   Epichlorohydrin/bisphenol A, an epoxy                                         resin sold by Shell as Epon 828.                                              MW = 380.                                                          EPOXY(2)   Epoxidized soybean oil, sold by                                               Viking Chemical Company as Vikoflex                                           7177. MW = 1000.                                                   EPOXY(3)   Epoxidized linseed oil, sold by                                               Viking Chemical Company as Vikoflex                                           7190. MW = 1050.                                                   EPOXY(4)   Butyl ester of epoxidized linseed oil                                         fatty acid, sold by Viking Chemical                                           Company as Vikoflex 9040. MW = 373.                                EPOXY(5)   Epoxidized octyl tallate, sold by                                             C. P. Hall Company as Monoplex S-73.                                          MW = 413.                                                          EPOXY(6)   Epoxidized glycol dioleate, sold by                                           C. P. Hall Company as Monoplex S-75.                                          MW = 637.                                                          ASTM       American Society Testing Materials                                 °F. Degrees Fahrenheit                                                 °C. Degrees Centigrade                                                 ______________________________________                                    

EXAMPLE I

Samples of 96% by weight PVDC-MA were mixed at room temperature with 4%by weight EPOXY for a total of 60 grams in a waring blender for 30seconds at approximately 300 revolutions per minute. Each blend wassubsequently charged to a Brabender Plasticorder mixing chamber whichwas heated to 320° F. (160° C.) by means of oil circulation between thechamber jacket and a Thermotron heat exchanger. Standard roller bladesrotating at a 3:2 drive: driven ratio with the driven roller rotating at63 rpm were used in all experiments. The thermal stability of each resinblend was determined by measuring the time within which the blendnoticeably darkened to a standard shade of brown. The results aresummarized in Table I below

                  TABLE I                                                         ______________________________________                                        INGREDIENTS                                                                                                         THERMAL                                 SAM-                                  STABILITY                               PLE   PVDC-MA   MW       EPOXY  MW    (MINUTES)                               ______________________________________                                        1     1         85,000   1      380   26                                      2     1         85,000   2      1000  20                                      3     1         85,000   3      1050  18                                      4     1         85,000   4      373   25                                      5     1         85,000   5      413   22                                      6     1         85,000   6      637   20                                      7     2         95,000   1      380   14                                      8     2         95,000   2      1000  13                                      9     2         95,000   3      1050  15                                      10    2         95,000   4      413   14                                      11    2         95,000   5      637   14                                      12*   3         105,000  1      380   13                                      13*   3         105,000  2      1000  13                                      14*   3         105,000  3      1050  12                                      15*   3         105,000  4      413   13                                      16*   3         105,000  5      637   12                                      ______________________________________                                         *Comparison of high MW PVDC                                              

As can be seen from comparison samples 12, 13, 14, 15 and 16, typicalthermal stabilities of 12 or 13 minutes were obtained with typical highmolecular weight PVDC of MW=105,000.

As can be seen from samples 7, 8, 9, 10 and 11, lowering the MW of thePVDC down to 95,000 resulted in a slight improvement in the thermalstability to 14 or 15 minutes.

However, as can be seen from samples 1-6, by decreasing the MW of thePVDC down to 85,000, a drastic improvement in thermal stability to 18 to26 minutes was observed. Moreover, of these six samples with low MWPVDC, those which had EPOXY also of low MW, namely sample 1 (Epoxy 1 hadMW=380) and sample 4 (EPOXY 4 had MW=373), were best with thermalstabilities of 26 and 25 minutes respectively.

EXAMPLE II

More PVDC samples are made as described in Example I, but this time thePVDC-MA is substituted with PVDC-VCl. The expected result is a similartrend in the thermal stability as with PVDC-MA.

EXAMPLE III

In a commercial-size blender, 480 pounds of PVDC-MA(1) was charged withinitial low speed blending at 115° F. (46.1° C.), and then added theretowas 20 pounds of EPOXY(2) with high speed blending at 200° F. (93.3°C.). Then, the blend was discharged to an after cooler at 105° F. (40.6°C.). This was 96% by weight PVDC-MA(1) and 4% by weight EPOXY(2). Fromit was made 4-layer, stretched oriented film by extrusion coating asdescribed in the laboratory examples of Brax et al U.S. Pat. No.3,741,253. First a 2-layer substrate of the structure: blend ofEVA+LLDPE/EVA was coextruded as a tube and then this was electron beamirradiated at about 4.5 MR. Then a layer of the blend ofPVDC-MA(1)+EPOXY(2) and a layer of a blend of EVA+LLDPE were extrusioncoated thereon so that the resultant from sealing layer to abuse layerhad the structure: 10% LLDPE+90% EVA/EVA/96% PVDC-MA(1)+4% EPOXY(2)/9%LLDPE+91% EVA.

Percentages noted were by weight. The tubular film was then biaxiallystretch oriented to a thickness of about 2.3 mils. The beforeorientation thickness of the PVDC-MA layer was about 2.2 mils and theafter orientation thickness was about 0.18 mils. The PVDC-MA layerexhibited excellent melt shear stability during extrusion. The resultantstretch oriented film had an excellent oxygen permeability of 4.05cc/mil/m² /day/atm, which is 0.26 cc/mil/100 in² /day/atm. Summarizedbelow are the results of various tests conducted on the biaxiallystretch oriented film.

    __________________________________________________________________________                          Tubular    Standard                                     Physical Property     Direction                                                                          Average                                                                             Deviation                                    __________________________________________________________________________    Tensile Stress at Break (psi)                                                                       L    8167  227                                                                T    9533  249                                          Elongation to Break (%)                                                                             L    207.9 5.8                                                                T    161.0 19.0                                         Modulus (psi)         L    32067 1693                                                               T    29667 1415                                         Free Shrink (%, at 165° F.)                                                                  L    13    1                                                                  T    27    2                                            Free Shrink (%, at 185° F.)                                                                  L    28    2                                                                  T    47    2                                            Free Shrink (%, at 205° F.)                                                                  L    54    1                                                                  T    64    1                                            Shrink Tension (psi, at 165° F.)                                                             L    155.48                                                                              20.07                                                              T    210.81                                                                              14.77                                        Shrink Tension (psi, at 185° F.)                                                             L    339.82                                                                              27.48                                                              T    339.82                                                                              27.48                                        Shrink Tension (psi, at 205° F.)                                                             L    229.21                                                                              7.70                                                               T    322.52                                                                              15.99                                        Energy to Break (ft-lbf/sq in)                                                                      L    1968.4                                                                              78.1                                                               T    1595.1                                                                              136.7                                        Tear Propagation (gram-force)                                                                       L    22.84 1.69                                                               T    51.99 33.19                                        Haze (%)                   3.4   0.6                                          Gloss (45 degrees)         90    3                                            Total Transmission (%)     91.2  0.2                                          Oxygen Transmission Rate (cc/sq m/day/atm)                                                               22.5  0.7                                          Saran layer gauge (mils)   0.18  0.01                                         Oxygen permeability (cc/mil/m.sup.2 /day/atm)                                                            4.05                                               Oxygen permeability (cc/mil/100                                                                          0.26                                               in.sup.2 /day/atm)                                                            __________________________________________________________________________     (1) OTR data was measured with the Macon OXTRAN at 73° F. and 0%       RH.                                                                           (2) All mechanical property data were obtained at 73° F.               (3) Tensile and elongation were as per ASTM D882.                             (4) Haze was as per ASTM D 1003.                                              (5) Gloss was as per ASTM D 523.                                              (6) Free shrink was determined by measuring unrestrained shrink at the        specified temp.                                                               (7) Shrink tension was determined using restrained shrink at the specifie     temp.                                                                         (8) Tear propagation was as per ASTM D 1938.                             

EXAMPLE IV

As in Example III, blends of PVDC-MA(1) were made but with EPOXY(4) andin 100 pound (220 kg) batches. Also, 100 pound (220 kg) batches ofPVDC-MA(3) with Epoxy (4) were made. From the blends were made 4-layer,stretched oriented films by extrusion coating as described in thelaboratory examples of Brax et al U.S. Pat. No. 3,741,253. First a2-layer substrate of the structure: blend of EVA+LLDPE/EVA wascoextruded as a tube and then this was electron beam irradiated at about4.5 MR. Then a layer of the blend of PVDC-MA+EPOXY(4) and a layer of ablend of EVA+LLDPE were extrusion coated thereon so that the resultantfrom sealing layer to abuse layer had the structure: 10% LLDPE+90%EVA/EVA/96% PVDC-MA+4% EPOXY(4)/9% LLDPE+91% EVA. Percentages noted wereby weight. The tubular film was then biaxially stretch oriented. Thebefore orientation thickness of the layers from sealing layer to abuselayer was about 3.5 mils/12.1 mils/2.2 mils/6.0 mils. The resultantstretch oriented films had a thickness of about 2.3 mils. The PVDC-MAlayer after orientation was about 0.18 mils. This PVDC-MA layerexhibited excellent melt shear stability and thermal stability duringextrusion. The following were made (percentages noted were by weight).

    __________________________________________________________________________                                     0.18 mil thickness                                                            PVDC-MA layer                                                      Oxygen Permeability                                                                      Oxygen Transmission Rate                     PVDC-MA(1)                                                                            PVDC-MA(3)                                                                            EPOXY(4)                                                                            *per m.sup.2 (100 in.sup.2)                                                              **per m.sup.2 (100 in.sup.2)                 __________________________________________________________________________    97%      0      3%    3.27 (0.21)                                                                              18.17 (1.17)                                 96%      0      4%    5.29 (0.34)                                                                              29.39 (1.90)                                 95%      0      5%    6.81 (0.43)                                                                              37.83 (2.44)                                 94%      0      6%    6.06 (0.39)                                                                              33.67 (2.17)                                  0      97%     3%    3.01 (0.19)                                                                              16.72 (1.08)                                  0      96%     4%    4.79 (0.31)                                                                              26.61 (1.72)                                  0      95%     5%    5.31 (0.34)                                                                              29.50 (1.90)                                  0      94%     6%    7.98 (0.51)                                                                              44.33 (2.86)                                 __________________________________________________________________________     *The first number shows the oxygen permeability as cc/mil/m.sup.2             /day/atm, and the second number with the parentheses shows the oxygen         permeability converted into cc/mil/100 in.sup.2 /day/atm.                     **The first number shows the oxygen transmission rate as cc/m.sup.2           /day/atm, whereas the second number with the parentheses shows the oxygen     transmission rate converted into cc/100 in.sup.2 /day/atm.               

EXAMPLE V

A 3-layer film is coextruded as a tube per the process described inBaird et al U.S. Pat. No. 4,112,181 having the structureEVA/PVDC-MA(1)/EVA wherein the PVDC-MA(1) core layer contains by weight1% Santicizer 141 (2-ethyl hexyl diphenyl phosphate) and 3% EPON 828(epichlorohydrin/bisphenol A). The film is biaxially stretch orientedand then electron beam irradiated at 1 MR. The before-orientationthickness of the PVDC layer is about 2.2 mils and the after-orientationthickness is about 0.18 mil, whereas prior art films are typically madewith a PVDC layer having a 3.0 to 3.5 mil before-orientation thicknessand 0.30 to 0.32 mils after-orientation thickness. The saran layerexhibits excellent melt shear stability during extrusion and minimaldiscoloration after irradiation, and oxygen barrier propertiescomparable to the thicker saran layer prior art film.

EXAMPLE VI

The process of Example V is repeated except this time PVDC-VCl,MW=85,000 is used instead of PVDC-MA(1). Similar oxygen barrierproperties, similar melt shear stability during extrusion and minimaldiscoloration after irradiation are exhibited, as for the film ofExample V.

EXAMPLE VII

layer film is made by extrusion coating as described in the laboratoryexamples of Brax et al U.S. Pat. No. 3,741,253, wherein first asubstrate of EVA is extruded as a tube and is irradiated by electronbeam at 4 MR and then by extrusion coating a layer of PVDC-MA(1) and alayer of EVA are added thereto so that the resultant is from sealinglayer to abuse layer of the structure: EVA/PVDC-MA/EVA. The PVDC-MA(1)contains by weight 1% Santicizer 141 (2-ethyl hexyl diphenyl phosphate)and 3% EPON 828 (epichlorohydrin/bisphenol A). The film is biaxiallystretch oriented and then is post-irradiated at about 1 MR. Thebefore-orientation thickness of the PVDC layer is about 2.2 mils and theafter-orientation thickness is about 0.18 mil, whereas prior art filmsare typically made with a PVDC layer having a 3.0 to 3.5 milbefore-orientation thickness and 0.30 to 0.32 mil after-orientationthickness. The saran layer exhibit excellent melt shear stability duringextrusion and minimal discoloration after irradiation, and oxygenbarrier properties comparable to the thicker saran prior art film.

EXAMPLE VIII

Example VII is repeated except with PVDC-VCl, MW=85,000 instead ofPVDC-MA(1). Similar oxygen barrier properties, similar melt shearstability during extrusion and minimal discoloration after irradiationare exhibited, as for the film of Example VII.

While certain representative embodiments and details have been shown forthe purpose of illustration, numerous modifications to the formulationsdescribed above can be made without departing from the inventiondisclosed.

What is claimed is:
 1. A method of improving the heat stability ofvinylidene chloride copolymer during tubular extrusion with a hot blownbubble, said method comprising extruding a film with a layer ofvinylidene chloride copolymer blend comprising:(a) about 0.01% to 6% byweight of processing aid wherein the processing aid possesses an epoxyfunctionality, and (b) low weight-average molecular weight vinylidenechloride copolymer wherein the major portion of the copolymer isvinylidene chloride.
 2. The method of claim 1 wherein the vinylidenechloride copolymer has a low weight-average molecular weight of about100,000 or less.
 3. In a method of improving the heat stability ofvinylidene chloride copolymer during tubular extrusion with a hot blownbubble followed by biaxial stretch orientation to make a heat-shrinkablefilm, said orientation being sufficient so that said heat-shrinkablefilm has a free shrink of about 5% or greater at a temperature of 96°C., the improvement comprising extruding a film with a layer ofvinylidene chloride copolymer blend comprising:(a) about 0.01% to 6% byweight of processing aid wherein the processing aid possesses an epoxyfunctionality, (b) low weight-average molecular weight of about 100,000or less vinylidene chloride copolymer wherein the major portion of thecopolymer is vinylidene chloride, and (c) the vinylidene chloridecopolymer blend layer has a thickness after orientation of less thanabout 0.29 mils.